Emulation circuits exist for emulating the characteristics of a component, such as a wire designed to carry high current loads. For example, an emulation circuit may be used to emulate the heating of a wire under a current load in order to generate a trip signal. The emulation circuit is often used to monitor the thermodynamics of a wire under a variable current load in order to identify abnormal conditions. A typical approach to modeling the thermal dynamics of a component, e.g. wire heating due to current, is to use a resistor capacitor (RC) time constant.
FIG. 1 is a circuit diagram illustrating one example of an emulation circuit that emulates the effect of a load current ILOAD on a wire connected to node X. This emulation circuit, typically referred to as an I2T timer, uses an RC time constant to set a threshold for an overload trip signal. In this example, resistor R and capacitor C are coupled to node X. ILOAD charges capacitor C and resistor R discharges capacitor C to obtain the RC time constant response characteristic. Comparator COMP compares the voltage level generated at node X to a reference voltage in order to generate a trip signal TRIP if the voltage level exceeds the threshold determined by reference voltage VREF.
The current ILOAD is a sample of the actual load current applied to the component to be emulated. For example, ILOAD may be a fractional representative sample of the actual load current that is squared, e.g. K*I2, and applied to the RC combination. The voltage at the RC combination is, therefore, representative of the power in the load. The transfer function for the thermal dissipation of the load is emulated through the selection of the RC time constant. For example, a thermal time constant of five seconds may be emulated using a RC combination of a 1 microfarad capacitor and a 5 mega-ohm resistor, which are generally large high-precision components.
Though large high-precision resistors are available, their effective resistance is often distorted by factors as humidity, induction or printed circuit board (PCB) surface leakage. Further, the electro-static discharge (ESD) diodes typically provided for the protection of integrated circuits (ICs) effect the resistive accuracy due to diode leakage at high temperatures. Also, large value precision capacitors are not readily available and those that are available are typically physically large. In addition, some types of high value capacitors, such as tantalum or electrolytic capacitors, have high levels of leakage, which also degrades the accuracy of an associated RC time constant. All of these factors are exacerbated for system components that may be required to operate in extreme environmental conditions, e.g. −55 to +125° C. temperature range, as well as widely varying humidity levels.